Roof underlayment rolls by area.

The underlayment calculator works out how many rolls of felt or synthetic underlayment cover your roof, based on area and roll coverage.

How to use this calculator

Roof area (sq ft)

Your true roof area including any waste allowance — underlayment covers the whole deck, so use the area you'll actually shingle.

Underlayment type

Synthetic (~1,000 sq ft/roll), 15 lb felt (~400 sq ft/roll), or 30 lb felt (~200 sq ft/roll).

How we calculate it

Rolls = roof area ÷ coverage per roll, rounded up. Coverage depends on the product: synthetic underlayment covers roughly 1,000 sq ft per roll, 15 lb felt about 400, and 30 lb felt about 200.

We round up because partial rolls aren't sold, and the overlap between courses (typically 2–4 inches) is already built into manufacturer coverage figures.

Underlayment goes over the entire deck regardless of shingle type, so the calculation uses total area, not bundle count.

A worked example

A 2,000 sq ft roof covered with 15 lb felt.

1. 15 lb felt covers about 400 sq ft per roll.
2. Rolls = 2,000 ÷ 400 = 5.
3. Rolls = ceil(5) = 5 rolls. (Synthetic at 1,000 sq ft/roll would be just 2 rolls.)

A 2,000 sq ft roof needs 5 rolls of 15 lb felt — or 2 rolls of synthetic, which covers far more per roll.

Frequently asked questions